A protective circuit comprised of a heater and low melting metal arranged on a substrate has so far been used as a protective circuit that is able to prevent not only over-current but also over-voltage and that may be used to advantage for secondary batteries for use in e.g., mobile electronic equipment.
In the protective circuit that protects the operation of the battery unit composed by a plurality of battery cells, there are occasions where shorting, for example, is produced in the inside of one or more battery cells to cause marked changes in the voltage within the circuit. If one or more of the battery cells is in disorder, the voltage of the battery unit is severely lowered, with the consequence that the heat quantity of a heating element is lowered so that the low melting metal may not be disrupted on melting. On the other hand, in case the battery unit voltage increases precipitously by some reason or other, the heating element emits excess heat, with the consequence that the heating element is scorched and hence falls into disorder so that the low melting metal may not be disrupted on melting.
To positively prevent the above mentioned damages to the heating resistance element to positively disrupt the low melting metal on melting, Patent Publication 1 discloses the following protective circuit. Viz., Patent Publication 1 discloses such a protective circuit in which, if a pre-set supply voltage is delivered thereto, the resistance value of a PTC, connected to the heating resistance element, is increased, with the current flowing through the heating resistance element being then lowered. It is possible in this manner to prevent large current from flowing through the heating resistance element to prevent it from scorching.